What will 4°C average global temperature mean for Planet Earth?

If no realistic agreement is made at the COP21 climate change conference in Paris, the world risks shooting past the safe 2ºC limit on its way to a 4ºC increase in average global temperature or more, but what will that mean for the planet?

A report produced by the Potsdam Institute for Climate Impact Research (PIK) and Climate Analytics for the World Bank in 2012 warned that such a temperature level would trigger a wave of cataclysmic effects including extreme heat waves, declining global food stocks and a sea-level rise that would adversely affect hundreds of millions of people. Every region would suffer, with the poorest suffering the most.

“A 4 degree warmer world can, and must be, avoided – we need to hold warming below 2 degrees” said World Bank Group President Jim Yong Kim at the time. “Lack of action on climate change threatens to make the world our children inherit a completely different world than we are living in today. Climate change is one of the single biggest challenges facing development, and we need to assume the moral responsibility to take action on behalf of future generations, especially the poorest.”

The report warned that a 4ºC scenario would be devastating, including inundation of coast cities, increasing risks to food production and many dry regions becoming dryer with other wet regions becoming wetter. There would be substantially exacerbated water scarcity in many regions of the world and an increased intensity of tropical cyclones. There would also be an irreversible loss of biodiversity, including coral reefs.

The largest warming would occur over land surfaces, ranging from 4° C to 10° C. Increases of 6° C or more in average monthly summer temperatures would be expected in the Mediterranean, North Africa, Middle East and parts of the United States. Sea levels could rise by 0.5 to 1 meter by 2100.

How would the climate change as the temperature approached 4ºC?

2°C

Once 2ºC is reached, preventing mass starvation in Africa will be very difficult, if not impossible, affecting billions of people. The last time the Earth reached such a temperature was in the Pliocene, the last epoch of the Tertiary Period, some 3 million year ago. Trees were growing in the Arctic at that time and there weren’t any glaciers on mountaintops. The sea level was 25 meters higher than it is today.

At such a temperature, the Amazon rainforest would die back and Greenland would melt. The sea wouldn’t be able to accept as much carbon dioxide as it does today, thereby causing a feedback cycle in which the carbon in the atmosphere would intensify climate change even further. The 1600 gigatons of carbon in the soil would be released into the atmosphere exacerbating the process even further.

3ºC

A 3ºC temperature could potentially be possible as early as 2050. The vegetation and the soil releases even more carbon, increasing atmospheric carbon by 250 parts per million by 2100. This in turn would increase the temperature by another 1.5ºC. This could potentially trigger a runaway effect, meaning that at this point climate change feeds itself and becomes impossible to stop. Cities like Houston could be destroyed by 2045, smashed by ‘super hurricanes’ and Australia would be uninhabitable. Eighty percent of sea ice will have melted.

4ºC

By now the rising temperature would have produced a constant stream of refugees fleeing from the coasts. Both polar ice caps would have melted and there would be a runaway thaw of permafrost. At this point, stabilizing global temperatures becomes impossible. The rainforests would have turned into desert and human society will have probably collapsed into civil war and chaos. Such a situation could be reality by the end of the century.

Avoiding a nightmare scenario

Fortunately, there are many grounds for optimism. The World Bank has found that more efficient use of energy would help to drastically reduce the impact of development on the climate without slowing poverty alleviation or impacting economic growth. Furthermore, all sorts of energy deals are now being concluded at the COP21 conference in Paris. Some experts are saying that it may be too late to avoid a 2ºC rise, but with a bit of luck, and some realistic policy decision making on the part of governments, we should be able to stop them rising before they get anywhere near 4°C.